Modify complex mesh to sotfen hard angles (eventually for 3D printing)
For fixing such models for 3D Printing I suggest taking a couple steps:


Use 'Tris to quads' (Ctrl + j in Edit mode)
Try selecting the edge loop in question (Alt + RMB)
By using 'Scaling' (s) and 'Moving' the object (g) in combination with 'Proportional editing' (o, use mouse wheel to specify the radius in which vertices will be affected) you can smoothen the area in question.


I also suggest using 'Smooth Vertex' located in the 'Tools' panel (t), only the selected vertices will be affected.

If you edit the mesh and reduce the amount of proximity loops around the edge you want to soften, you can use subdivision modifier.

If you can't avoid the overhangs (front of the mask rounded, and the back has steep angles as in the picture you attached) you might want to split the model into parts and print them in a different orientation, in most cases however it requires more work to choose this solution.

Lastly, since the overhangs shown in the picture you attached are quite steep. Unless you want to make the back completely flat, I would suggest you still use some supports to print such models (as the overhangs tend to start at the last layers of the print, you might be disappointed to see the model printed well and when it came to the overhangs layers it failed.)